fix(code-quality): Fix for Explicit returns mixed with implicit (fall through) returns

[jackseceng]

To fix this, make input_url() total: every control-flow path must explicitly return a response or raise via abort.
Best minimal fix without changing behavior: add a final fallback return after the match in input_url() that returns the existing 500 handler response. This prevents implicit None return if any path in the "POST" branch ever falls through.

Edit in app/app.py, inside input_url():

• Keep existing logic unchanged.
• Replace the unreachable return None after abort(...) in case _ with a concrete response return (or remove it), and
• Add/retain a final explicit return internal_server_error(HTTPStatus.INTERNAL_SERVER_ERROR) at function end so no implicit return is possible.

No new imports or dependencies are needed.

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